1. Field of the Invention
The present invention relates to an IC package having a lot of pins, and an optical receiver and an optical transmitter each of which employs the IC package.
2. Description of the Prior Art
Referring now to FIGS. 13A to 13C, there is illustrated a prior art IC package having a lot of pins and including such a circuit as a multiplexer (MUX) for use in an optical transmitter or a demultiplexer (DEMUX) for use in an optical receiver. FIG. 13A is a top plan view of the IC package, FIG. 13B is a side view of the IC package, and FIG. 13C is a bottom plan view of the IC package.
In FIGS. 13A to 13C, reference numeral 1 denotes a chip carrier constructed of low temperature cofired ceramic in which a plurality of dielectric substances each of which includes conductive pads, vias (or connection holes), conductive lines, and grounded conductors are laminated. For example, the chip carrier 1 has a 10 mm to 20 mm square. Reference numeral 2 denotes a group of one or more bare chips mounted on an upper surface of the chip carrier 1, the group having a function of serving as MUX or DEMUX in the unit of the group of one or more bare chips and each bare chip having a 1 mm to 5 mm square, reference numeral 3 denotes a metallic lid for covering the group of one or more bare chips 2 and so on, and reference numeral 14 denotes a bump (projecting electrode) with a size of about 0.5 mm, which constitutes a ball grid array (BGA), a plurality of bumps 14 being disposed on a lower surface of the chip carrier 1. In general, a plurality of bare chips 2 are mounted on the chip carrier 1.
FIG. 14 is a side view showing the structure of either an optical transmitter or an optical receiver which employs the IC package shown in FIGS. 13A to 13C. In FIG. 14, reference numeral 4 denotes a circuit board made of a resin and equipped with the IC package 20 consisting of the chip carrier 1, the plurality of bare chips 2, the metallic lid 3, and the plurality of bumps 14, reference numeral 30 denotes an optical module that performs electric-to-light conversion or light-to-electric conversion, reference numeral 32 denotes a feedthrough disposed in the optical module 30, through which high-frequency signals are transmitted, and reference numeral 45 denotes leads for connecting a differential pair of lines disposed on the circuit board 4, through which high-frequency signals are transmitted, to a differential pair of lines formed in the feedthrough 32.
In the case of the optical transmitter, the IC package 20 shown in FIG. 14 has the group of bare chips 2 having a function of MUX, and the optical module 30 can be either an LD (laser diode) module provided with a laser diode for generating an optical signal from an electrical signal or an EA module provided with an EA element (electro-absorption modulator) for generating modulated light from incoming light of certain intensity. In contrast, in the case of the optical receiver, the IC package 20 has the group of bare chips 2 having a function of DEMUX, and the optical module 30 can be a PD (photo diode) module provided with a photo diode for generating an electrical signal from an optical signal.
MUX used for optical transmitters and DEMUX used for optical receivers are high-performance ICs, and each of them consists of high-density MMICs (monolithic microwave integrated circuits). MUX has a signal multiplexing function and converts a 16-channel parallel data signal of 2.5 Gbps which is a low-speed parallel data signal from a logic LSI on a system's side into a 1-channel serial data signal of 40 Gbps which is a high-speed serial data signal. MUX then outputs the serial data signal to a laser diode or an EA element in the optical module 30 by way of the feedthrough 32.
DEMUX has a signal demultiplexing function and converts a 1-channel serial data signal of 40 Gbps which is a high-speed serial data signal and which is input from a photo diode or the like in the optical module 30 by way of the feedthrough 32 into a 16-channel parallel data signal of 2.5 Gbps which is a low-speed parallel data signal. DEMUX then outputs to the 16-channel parallel data signal to a logic LSI on a system's side.
Thus MUX and DEMUX are high-performance ICs with a lot of pins. Accordingly, QFP (quad flat package) or BGA (ball grid array) is generally adopted as the structure of an IC package housing such a high-performance IC. BGA is preferable to achieve a higher density.
However, when such an IC package structure is adopted, it is difficult to transmit a wide-band signal including high-frequency components such as microwaves or miliwaves over a wide frequency band from DC level to several tens of GHz with slight deterioration of the transmission characteristics of high-frequency signals. Such a transmission technology indispensable to implement 40 Gb/s optical transmitters and 40 Gb/s optical receivers has not been established. For example, when connecting the IC package 20 having a 10 mm to 20 mm square to the circuit board 4 by using BGA, each bump 14 has to have a size of about 0.5 mm in order to ensure that each bump 14 disposed on the lower surface of the IC package 20 is secured to the circuit board 4 by soldering.
However, with the current state of the art, when the IC package 20 is secured to the circuit board 4 by using the plurality of bumps 14 each having such a size, only high-frequency signals of about 20 Gbps or less (microwave band) can be input and output to and from the IC package 20. This is because the change in the characteristic impedance according to a stray capacitance between the plurality of bumps 14 and grounded conductors increases as the frequency rises, and the transmission characteristics of high-frequency signals are therefore deteriorated.
Furthermore, as shown in FIG. 14, in the conventional example of implementation of BGA, the differential pair of lines disposed in the feedthrough 32 of the optical module 30 are electrically coupled to the differential pair of lines extending to an end portion of the upper surface of the circuit board 4 by way of the leads 45 so that the circuit board 4 and the optical module 30 are electrically connected to each other. The junction between the differential pair of lines in the optical module 30 and the differential pair of lines on the circuit board 4 thus has a discontinuous point at high frequencies, and further transmission characteristic deterioration caused by this discontinuous point is not avoided. Furthermore, since the transmission path for high frequencies becomes long by the connection length L1 of the leads 45 and the line length L2 from the junction to the BGA and the transmission loss increases with the increase in the length of the transmission path, the transmission characteristics of high-frequency signals is further deteriorated.
On the other hand, as a general technique for transmitting high-frequency signals in the IC package 20, a feedthrough structure using a thick ceramic circuit board or the like is often used. When all interfaces of the IC package 20 having a lot of pins, such as MUX or DEMUX, are constructed of feedthroughs, thick ceramic circuit boards are arranged on lateral surfaces of the IC package 20 so that they are projecting in a horizontal direction from the lateral surfaces, and a plurality of leads, which penetrate the IC package 20, are arranged on each thick ceramic circuit board. Therefore, the IC package 20 increases in size and wire bonding or ribbon bonding is needed for the connection between the plurality of leads and an external circuit, thereby deteriorating the manufacturability.
In sum, the first problem with the prior art IC package 20 constructed as mentioned above is that when all input/output terminals of the IC package 20 with a lot of pins, such as MUX and DEMUX, are constructed of BGA and high-frequency signals are transmitted on the circuit board 4, the deterioration of the transmission characteristics of high-frequency signals increases according to the change in the characteristic impedance between each bump 14 and the grounded conductors, the discontinuous point at high frequencies in the junction between the circuit board 4 and the optical module 30, and the transmission loss caused in the transmission path between the IC package 20 and the optical module 30.
Furthermore, the second problem with the prior art IC package 20 is that when all input/output terminals of the IC package 20 with a lot of pins, such as MUX and DEMUX, are constructed of feedthroughs which provide slight deterioration of the transmission characteristics of high-frequency signals, the IC package 20 increases in size and a plurality of leads or the like are needed for the connection with an external circuit, thereby deteriorating the manufacturability.